NETWAS Uganda
LeaPPS Uganda
Learning for Practice and Policy
on Household and School
Sanitation & Hygiene
Action – Research
at household,
community and
school level project
Support input by IRC, Jo Smet
Quality Assurance by Brenda Achiro
Version-5: April 2009
GUIDELINES FOR BUILDERS/MASONS ON ARBORLOO AND FOSSA ALTERNA
Key source: ‘Toilets that make compost’, Peter Morgan www.ecosanres.org/toilets_that_make_compost.htm
Introduction Arborloo (Tree Latrine)
An Arborloo is a ‘Tree Latrine’. It is a
shallow composting latrine. The magic
comes when you add soil, ash and
hopefully leaves into the pit as well as
excreta. These extra ingredients
accelerate the composting process and
make what goes on below ground level
rather different from the standard pit
toilet. In a period of 9-12 months the
pit fills up. Then a new pit has to be
dug. The latrine house, the concrete
slab and the ring beam or brick collar
move all to the new pit. A new
Arborloo is then ready for use. The old latrine is covered with soil and a tree is planted. There is no need to
touch the partly decomposed faeces! So it is safe! Any tree will grow on the rich compost; fruit trees,
bananas and papayas are very suitable and commonly planted. But also trees used for timber can be
planted. Every year the household has a new tree, which gives fruits or timber for the family and excess
fruits can be sold for extra income. Apart from having a growing orchard of trees, the Arborloo has more
advantages. Because the pit is shallow, it is a sound solution for (i) high groundwater tables, (ii) rocky soils;
and (iii) sandy (loose) soils. And because the latrine collar is placed above ground level, the slab is some 3040 cm above the ground, and that may be sufficient to avoid seasonal flooding/overflowing in flood-prone
areas.
Introduction Fossa Alterna (Twin Composting Latrine)
A Fossa Alterna or Twin Composting
Latrine and its composting process are
based on the same principle as for the
Arborloo: a shallow composting
latrine. But has a more permanent
structure. Where you do not have
sufficient space to dig a new pit and
shift the latrine every year, the double
(alternating) pit option is the solution.
If the first pit is full after 12 months of
use, one transfers latrine house and
slab to the second pit. Then when the
second pit is about to be full, the first
pit will be emptied. The safe manure
(also called humanure) can be used in
the garden. It can also be sold to
farmers (provided they are
interested). The Fossa Alterna is therefore a more permanent structure. Because the pit is shallow, it is a
sound solution for (i) high groundwater tables, (ii) rocky soils; and (iii) sandy (loose) soils. Does the name
‘Fossa Alterna’ sound strange, you can give this option a good local name? Composting latrine, or ‘Manure
latrine’?
Guidelines for Builders Arborloo-Fossa Alterna
Jan-09, Jo Smet-IRC
Page 1 of 9
NETWAS Uganda
LeaPPS Uganda
Learning for Practice and Policy
on Household and School
Sanitation & Hygiene
Action – Research
at household,
community and
school level project
The latrine components
The latrine house:
Can be made of any material that is available.
For instance, from poles and grass; from poles and mud; from poles or timber
and papyrus stalks; all timber; poles and corrugated iron sheets; masonry; ferrocement etc. The choice of building materials has a direct effect on the cost!
The slab or floor:
Must be made of reinforced ‘concrete’; the concrete is made of coarse (river or
builders) sand and cement with iron wire reinforcement
The ring beam or collar
Can be made of reinforced ‘concrete’ then it called a ring beam; a ‘collar’ is made
of burnt bricks with mortar, sun-dried bricks with termite hill soil, or stones/
rocks/pebbles/large gravel kept together with cement mortar or termite hill soil.
The pit
Does not need any lining provided the soil is not too coarse (e.g. very coarse
sand or fine gravel) and tends to collapse, and the pit is shallow.
Steps in constructing an Arborloo or Fossa Alterna
Step 1: Locate the building site of the EcoSan latrine
 Stay at least 5 m away from the kitchen (for pit latrines officially 30m from home)
 Stay at least 10 m away from any dug well (if present) (for pit latrines officially 30m)
 If groundwater level is high (less than 1m), build latrine at higher ground level
Step 2: Prepare building site of the latrine
 If the collar option is chosen, level the ground (do not dig the pit yet!)
 If the ring beam option is chosen, raise the ground level some 10cm and ram the fresh soil.
Level the raised surface.
Step 3: Make the concrete slab
 Can be circular or square (dimensions depending on pit size – see later in text)
 Can be cast in a timber frame or in a brick frame (or even metal frame)
 Accurately level the ground where you will cast the slab
 Always put plastic or paper on the ground before casting to avoid slab sticking to ground
 Best to apply some used engine oil to timber/brick frame and hole mould
Step 4: Make collar or ring beam at the site
Step 5: Dig latrine hole to required depth
Step 6: Place concrete slab on concrete ring beam or brick collar
Step 7: Build superstructure (latrine house)
Step 8: Empty a full sack of dry leaves to the pit
The pit
The pit size
The size or volume of the pit is important as the period of use of latrine should be at least 9-12 months for
the Arborloo and at least 12 months for the Fossa Alterna. The 12 months are needed to make the compost
safe so that no pathogens survive that could create a health risk. But making the latrine last for more than
12 months does not make sense. Only the construction cost will go up steeply.
An individual may produce some 60 litres of dry faeces per year (small kids less and perhaps adults more).
The dry soil, ash and leaves added to the pit may add another 60 litres per person per year. We add another
20% volume as a ‘safety’ buffer (the factor 1.2 in the calculations below).
So the total required volume for a family of five people is estimated at:
5 x (60+60) x 1.2 = 720 litres
For a family of 10 people:
10 x (60+60) x 1.2 = 1,440 litres
Guidelines for Builders Arborloo-Fossa Alterna
Jan-09, Jo Smet-IRC
Page 2 of 9
NETWAS Uganda
LeaPPS Uganda
Learning for Practice and Policy
on Household and School
Sanitation & Hygiene
Action – Research
at household,
community and
school level project
For a school latrine used by 60 students, the calculation has to take into account that perhaps not more than
one out of three pupils will use the latrine for defecation, and only five days a week and 40 weeks a year
(some 12 weeks school holidays); so the total required volume is estimated at:
(60:3) x (60+60) x (5/7) x (40/52) x 1.2 = 1,582 litres
Furthermore, the pit dimensions are depending on – these are important rules:
1. The groundwater level; have bottom of pit always above the highest groundwater level. Faeces
decomposes differently in water, giving bad smells, attracting flies and mosquitoes etc. This means
if the groundwater reaches 0.5 m below ground level and you need a pit of 1.0 m deep, then you
have to raise your pit by building a higher collar (pit walls) on which you put your slab; or find
another higher location for the latrine.
2. The soil texture – 1;
if too hard (rocky) to dig, then you need to build the pit partly or fully above the ground.
3. The soil texture – 2
if soil is very loose (sandy), then the pit can not be too deep (for instance not more than 70-80 cm),
and a round pit is stronger than a square or rectangular pit
4. The construction costs of ‘concrete’ slab
if width & length or diameter (circular slab) are more than 1.1 m (pit size 1.0m) then more
reinforcement is needed (at least two iron wires), and then cost may become high
5. The use of a concrete ring beam, stone collar or brick collar
the height of a concrete ring beam may be around 10-12 cm. That of a stone/rock/boulder collar
some 15 cm, and that of a brick collar some 35-40 cm. These beam and collars are placed above the
ground level. In use, the pit will be filled up to a level of some 35 cm below the slab. For the brick
collar that is up to the original ground level, for the ring beam and rock collar that is some 20-25 cm
below ground level. This 25 cm needs to be added to the pit depth as it is not effective pit depth
(not to be filled up).
The pit dimensions for Fossa Alterna (12 months -use) are given in table below. The most recommended
choices are made green. The slab sizes are 10 cm larger/wider - to have 5 cm overlap on brickwork.
Users per
latrine
School: 60
School: 60
School: 60
HH: 10
HH: 5
Pit volume
[litres]
1,582
1,582
1,582
1,440
1,440
1,440
720
Depth
[m]
1.0
1.25
1.50
1.0
1.25
1.50
0.8
Circle diam. [m]
1.40
1.30
1.15
1.35
1.20
1.10
1.10
Square
[m]
1.25
1.15
1.05
1.20
1.05
1.00
0.95
Rectangular
Width [m]
0.70
0.70
0.70
1.00
1.00
1.00
1.00
Rectangular
length [m]
2.25
1.80
1.50
1.45
1.15
0.95
0.90
Remark: possible depth depends also on groundwater table and soil texture- see above
The pit dimensions for Arborloo (12 months -use) are given in the table below. The most recommended
choices are made green. The slab sizes are 10 cm larger/wider - to have 5 cm overlap on brickwork.
Users per
latrine
School: 60
School: 60
School: 60
HH: 10
HH: 5
Pit volume
[litres]
1,582
1,582
1,582
1,440
1,440
1,440
720
Depth
[m]
1.0
1.25
1.50
1.0
1.25
1.50
0.8
Circle diam. [m]
of pit
1.40
1.30
1.15
1.35
1.20
1.10
1.10
Square
[m] of pit
1.25
1.15
1.05
1.20
1.05
1.00
0.95
Rectangular
Width [m]
Not applicable
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
Rectangular
length [m]
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
Remark: possible depth depends also on groundwater table and soil texture- see above
The pit dimensions for Arborloo (9 months -use) are given in the table below. The most recommended choices
are made green. The slab sizes are 10 cm larger/wider - to have 5 cm overlap on brickwork.
Users per
latrine
School: 60
Pit volume
[litres]
1,187
HH: 10
1,080
HH: 5
540
Depth
[m]
1.0
1.25
1.50
0.9
1.00
1.25
0.60
Circle diam. [m]
1.25
1.10
1.00
1.25
1.15
1.05
1.10
Square
[m]
1.10
1.00
0.90
1.10
1.05
0.95
0.95
Rectangular
Width [m]
Not applicable
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
Rectangular
length [m]
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
Remark: possible depth depends also on groundwater table and soil texture- see above
Guidelines for Builders Arborloo-Fossa Alterna
Jan-09, Jo Smet-IRC
Page 3 of 9
NETWAS Uganda
LeaPPS Uganda
Learning for Practice and Policy
on Household and School
Sanitation & Hygiene
The Pit shape
Circular
Circular shape is strongest. This is
best solution in loose soils and
high groundwater tables, as shape
gives most stable pit walls.
Circular shape can withstand
higher soil forces.
Square
Square shape is OK in
more stable soils with
deeper/low
groundwater tables.
Action – Research
at household,
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Rectangular
Rectangular shape is sound for higher pit
volumes and where soils are more stable
(or with lining). The latrine house will be
built above part of the pit. The part outside
the latrine house could be used for
emptying the pit.
The slab
The shape and mould
Guidelines for Builders Arborloo-Fossa Alterna
Jan-09, Jo Smet-IRC
Page 4 of 9
NETWAS Uganda
LeaPPS Uganda
Learning for Practice and Policy
on Household and School
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community and
school level project
The circular slab has a diameter of the pit size plus
10 cm, to allow for 5 cm overlap at all sides.
Thickness of the slab is 40 mm provided coarse
river sand and fresh cement is being used. If
quality of sand and/or cement is poor, 45-50mm to
be at safe side (worst ‘business’ scenario is a slab
collapsing with someone ending in the pit, even if
the pit is quite shallow and dry!)
The square slab has a size of the width plus
10 cm, to allow for 5 cm overlap at all sides.
Thickness of the slab is 40 mm provided
coarse river sand and fresh cement is being
used. If quality of sand and/or cement is
poor, 45-50mm to be at safe side (worst
‘business’ scenario is a slab collapsing and
the user ending in the pit!)
The mould can be made of bricks of 40 mm height
(for slab of 40 mm thickness; if slab is thicker use
bricks with height accordingly. Making a circular
timber or metal mould is attractive and
worthwhile if expected number of slabs to be
produced is high.
The square mould can be made of bricks or
timber of 40mm x 40mm. The rectangular
corners can be fixed as above.
Put a sheet of plastic or paper at the bottom
to prevent that slab sticks to ground.
The mould and slab casting
Squatting hole
To avoid soiling and urine splashing on the slab, the
squatting hole should have an appropriate shape.
The hole should also not too big as small kids may
fear to fall in the ‘dark’ hole. A common shape is
the key-hole; it provides sufficient space for
urination. It is basically a circle and an ellipse. The
diameter of the circle is 15cm and the ellipse is
20cm, see figure next
The mould for the squatting hole can be made of an
empty paint tin or a small bucket, or a pan; and a
smaller round object (diameter some 10cm).
For more permanent moulds, one could use a
timber, metal or masonry shaped mould. Always
apply some used engine oil to the mould to prevent
the concrete sticking to the mould.
Concrete mixture
Some general advice:
 Use coarse sand, river sand, also called builders sand. If sand is fine, the concrete will not be strong!
 Use clean sand. Always wash the sand and remove silt, organic material etc.
 Use fresh cement; old cement has lost much of its strength; old cement tends to granulate and then
mixing with sand is also not optimal
 Do not use gravel; the thickness of the slab (40-50mm) is too small for such large aggregates
 Mix sand and cement thoroughly by turning it many times until the product has a uniform grey colour
 Add water slowly and mix continuously
 Do not add too much water, usually not more than half the volume of cement used
 The final product should be a stiff mixture
 When casting the slab, first put a bottom layer of ‘concrete’ of 10-15 mm, then put the reinforcement,
and then add concrete to fill the mould; level off and smoothen with a wooden float
 After casting, at least 10 days are needed for curing. Keep the slab always wet and have it covered with
gunny (jute) bags or grass or banana leaves to protect it from the direct sunlight!
 Concrete will reach its maximum strength 28 days after casting.
A good mixture is 1:3, that is one volume of cement to three volumes of sand. For a slab of 40mm thick and
a size 1.00m x 1.00m, 30 litres of washed coarse sand and 8-10 litres of fresh cement would do. Just try it
Guidelines for Builders Arborloo-Fossa Alterna
Jan-09, Jo Smet-IRC
Page 5 of 9
NETWAS Uganda
LeaPPS Uganda
Learning for Practice and Policy
on Household and School
Sanitation & Hygiene
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at household,
community and
school level project
and adjust if needed. As ‘seeing is believing’, have the finished/cured slabs loaded with some 160 kg (= two
adults) to test the strength of the slab!
The ring beam or collar of burnt bricks, sun-dried bricks or rocks/pebbles/gravel
The circular brick collar
1. Mark a circle on the ground with the
diameter required for the pit
2. Lay burnt or sun-dried bricks around the
circle
3. Make a good mortar of ant hill soil and
water (for the Fossa Alterna, with a
permanent collar, also cement mortar can
be used) (the Arborloo collar will be
dismantled when shifting)
4. Add the ant hill or cement mortar between
and above the bricks
5. Add another two courses of burnt bricks in
the same way (joints of layer 1 and 2 not
above each other!)
6. Add extra mortar above course three to get
levelled surface
The circular concrete beam
1. Level off some ground and lay plastic sheet
over the ground
2. Take some bricks of 10-12 cm high
3. Lay the bricks so that the outer and inner
circles will make a ring beam of 15 cm in
between them; the beam is 15cm all around
4. Fill the spaces opened up between the inner
bricks with wet sand
5. Put plastic sheet inside to prevent concrete
sticking to the bricks
6. Put concrete mixture (1:3, see above) on the
bottom of the mould (fill some 2-3cm)
7. Take a length of wire of 3-4mm diameter and
place it on the concrete bed, half way between
inner and outer bricks
8. Then add remainder of the concrete mix
9. Level off with a wooden float
10. Ram hard down the wooden float
11. Steel handles can be added if required
12. Cure concrete ring beam by covering and
keeping it with for at least 10 days
For the Arborloo, instead of burnt or sun-dried
bricks, rocks or pebbles mixed with a rich anthill
mortar can be used.
Make sure the shape of the ring beam is correctly
shaped with a width of some 15 cm.
Guidelines for Builders Arborloo-Fossa Alterna
Jan-09, Jo Smet-IRC
Page 6 of 9
NETWAS Uganda
LeaPPS Uganda
Learning for Practice and Policy
on Household and School
Sanitation & Hygiene
The square brick collar
The construction is similar as for the circular brick
collar, only the shape is different. Make sure the
inner dimensions are the same as these of the pit to
be dug.
Below a picture of a square brick collar with
cement mortar. Remark: instead of cement mortar,
termite hill soil might be used.
Action – Research
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The square concrete beam
The construction is similar as for the ring beam,
only the shape is different. Make sure that the
width is some 15cm and the height some 10-12 cm,
and do not forget the reinforcement wire.
Below a picture of a fully lined pit needed when soil is
very loose or when a masonry latrine house will be
built on top.
Fossa Alterna design of rectangular pits
Rectangular pits of Fossa Alterna are suitable for schools and larger households; see also table with
dimensions. School pits require a high pit volumes. Therefore check to what depth you can dig before you
reach the ground water; if 1.50m is possible, fine. Stability of the soil is less an issue as the pits will be fully
lined. If max. depth is only 1.00m then the surface area of the pit will be larger and the length greater. For
schools latrines that may require a permanent superstructure of bricks, the pit needs to be lined up to the
bottom to avoid pit collapsing.
Guidelines for Builders Arborloo-Fossa Alterna
Jan-09, Jo Smet-IRC
Page 7 of 9
NETWAS Uganda
LeaPPS Uganda
Learning for Practice and Policy
on Household and School
Sanitation & Hygiene
Action – Research
at household,
community and
school level project
To save construction cost, pit and superstructure walls may be shared. That means that pits needs to be
narrow (width of 70cm) and long enough to have an external opening for emptying (length > 1.50m). That
length of the pit, however, demands for extra attention during operation as fresh compost needs to be
levelled regularly over the entire pit surface. The drawing below shows a four-stance Fossa Alterna for
schools. The superstructure is built over the walls and the concrete beam. The rear of the pit is closed with
a removable light concrete slab (with metal wire hooks), sealed with termite hill soil. A beam crossing all
pits makes building of the superstructure/house possible (dimension house 1.00mx1.50m). The beam
should be at least 120mmx120mm with 2 iron reinforcement bars of at least 10mm diameter place low.
A front wall may replace doors (always problematic) and private still enough privacy.
For larger households the Fossa Alterna may be built adjacent (see below) that allows a permanent
superstructure (over two beams), or apart (see below). In the latter case the transferable superstructure is
also partly build on the beam, and a light slab covers the rear hole for emptying. Spreading /levelling of the
excreta/compost (also from the rear hole) in the pit is needed to have a proper functioning of the Fossa.
The latrine house or superstructure
The options and implications
 Poles and grass, papyrus, palm leaves or banana leaves
Cheap and easy to transfer or replace. Make sure that the poles
are put in the ground outside the ring beam or square collar by
digging/boring small diameter holes (not making pit to cave in)
 Timber or metal frame with grass, papyrus, palm leaves or
banana leaves
Advantage is that the ‘house’ can easily be transferred to new
Arborloo or Fossa Alterna. More expensive because of cost of
timber or metal frame
 Fully made of timber: More expensive and heavier but still
transferable
 Poles and mud: cheap though heavier and not easily
transferable
 Fully made of burnt bricks: Heavy and expensive. This requires
a full lining of pit otherwise the pit will collapse quickly!
 Roofing material: any in principle, that is, corrugated sheet
(expensive but durable), papyrus (cheap if available), grass
(cheap), palm or banana leaves (cheap); however, corrugated
sheet are durable.
Guidelines for Builders Arborloo-Fossa Alterna
Jan-09, Jo Smet-IRC
Page 8 of 9
NETWAS Uganda
LeaPPS Uganda
Learning for Practice and Policy
on Household and School
Sanitation & Hygiene
Action – Research
at household,
community and
school level project
Urinals for boys and girls – see Guidelines for Builders/Masons on Urinals
Guidelines for Builders Arborloo-Fossa Alterna
Jan-09, Jo Smet-IRC
Page 9 of 9